1. Field of the Invention
The present invention relates to a measurement apparatus, a measurement method, and an absolute encoder.
2. Description of the Related Art
Conventionally, an incremental encoder or absolute encoder has been used to measure a position and angle. Incremental encoders record a grating of a given period in a scale or disk, optically or magnetically read the movement of the grating, and calculate an absolute position in combination with an origin detection mechanism. In recent years, there have also been high-resolution products, each of which has a grating pitch decreased to about 80 μm and interpolates phase information within one pitch by dividing it into about 10,000 sections using an electrical divider. Also, since recent incremental encoders read grating lines by optically averaging them instead of reading the grating lines one by one, a pattern error of the grating itself is canceled, resulting in high accuracy. Since, however, the incremental encoders cannot obtain absolute position information unless the origin is initially detected, applications particularly to the fields of machine tools and robots are limited.
On the other hand, absolute encoders can output absolute position information instantly by reading a binary pattern such as a gray code or M-sequence code as an image using a light-receiving element array or an image sensor such as a CCD. Japanese Patent Laid-Open No. 2004-529344 discloses a coding method as an absolute coding method. In Japanese Patent Laid-Open No. 2004-529344, an absolute code is embedded by, for example, removing, among the reflective gratings and non-reflective gratings of an incremental encoder, some reflective gratings, and a code and phase information are calculated using image information (pattern boundary information) from a light-receiving element array.
In Japanese Patent Laid-Open No. 2004-529344, however, an absolute code and interpolation phase information are calculated by a pattern matching (correlation calculation) method of shifting (laterally shifting) the phases of an image and reference table data by paying attention to the boundary position of a density pattern. In Japanese Patent Laid-Open No. 2004-529344, therefore, the resolution is basically limited by the precision of an imaging optical system and a light-receiving element array. Furthermore, in Japanese Patent Laid-Open No. 2004-529344, a large-scale calculation electronic circuit for image processing is required, and it takes time to calculate absolute position information due to complicated calculation.